Posted
by
kdawson
on Wednesday November 04, 2009 @09:51AM
from the patchin'-place dept.

Red Midnight and other readers brought to our attention a bug in most deployed versions of Linux that could result in untrusted users getting root access. The bug was found by Brad Spengler last month. "The null pointer dereference flaw was only fixed in the upcoming 2.6.32 release candidate of the Linux kernel, making virtually all production versions in use at the moment vulnerable. While attacks can be prevented by implementing a common feature known as mmap_min_addr, the RHEL distribution... doesn't properly implement that protection... The... bug is mitigated by default on most Linux distributions, thanks to their correct implementation of the mmap_min_addr feature. ... [Spengler] said many other Linux users are also vulnerable because they run older versions or are forced to turn off [mmap_min_addr] to run certain types of applications." The register reprints a dialog from the OpenBSD-misc mailing list in which Theo De Raadt says, "For the record, this particular problem was resolved in OpenBSD a while back, in 2008. We are not super proud of the solution, but it is what seems best faced with a stupid Intel architectural choice. However, it seems that everyone else is slowly coming around to the same solution."

Is it possible that you are running wine or some other emulator program. The only software similar to an emulator I have is Virtualbox on my 9.10 desktop and it still has the 65536 setting.
Anyone else can shed light on this?

I have two machines with Open SuSE 11.1 One returns 0, and the other 65535. The one returning zero runs only samba and apache (+webdav module for an ical server). The one returning 65535 runs samba and xen.

No, this isn't the same bug. People confuse two issues. I wrote the mmap_min_addr protections to try to mitigate the effects of a certain class of common kernel bugs which exist because of design choices by Intel. That class of bugs can be summed up as NULL pointer usage. Every time someone finds a new NULL pointer usage bug we get the same story. RHEL (and any system with SELinux enabled) did not have protections for mapping the 0 page by local authenticated users, but did have protections for network facing daemons and the like. Other distros had protections for the local authenticated user but weaker protections for network facing daemons. The mmap_min_addr protections have since been enhanced in SELinux systems such that they have stronger protections, both for local authenticated users and for network facing daemons. My old comments from the first time this came up are at http://eparis.livejournal.com/ [livejournal.com]

But the key to remember is that mmap_min_addr implementation is not the bug that allows elevation of privilege. In this case it was a very very old bug in the implementation of pipes. Previously Spender and friends have found bugs in performance counters (one which was actually much worse as it didn't fit into the very narrow class which might be mitigated by mmap_min_addr), in network sockets, and other places. These are the bugs which cause this to be a new story. Once he finds the real bugs he applies some of the same basic techniques (plus a whole lot of thought) to create an exploit. If the Linux kernel was bug free we wouldn't need mmap_min_addr. If mmap_min_addr was bug free (over the years Spender has found multiple problems with my work) this class of bugs would be just a bit less devastating.

Everyone in the kernel development community needs to think of invalid pointer bugs as a larger security threat then they currently do. The lesson here, keep your systems patched.

My understand in a short and simple answer: before the x86_64 architecture revolution it was possible to effectively have a separate virtual memory space for the kernel and userspace. With x86_64 segmentation was removed.

Intel i686 processors had a pretty big performance hit if you used functionality since there was a TLB flush on every kernel trap. I don't believe many kernels made use of it. The PaX security patch set could and Red Hat's 4g/4g split kernels did (RH called them hugemem kernels, although I don't suggest them to anyone as they had their own issues.) The AMD Opteron processors actually had tagged TLBs so they didn't have to flush and performance didn't suffer nearly as badly.

In any case, with x86_64 that ability is simply gone. SPARC is often named as an architecture which doesn't suffer from this specific class of bugs since it always had completely separate kernel space and userspace virtual memory. I heard rumour their might be a brilliant solution that will hopefully come from the PaX/grsecurity team, but I don't believe anything has materialized just yet.

Most people expect that dereferencing a NULL pointer will result in some sort of fault. If you map the virtual page 0 and then do this you don't get a fault. Remember that NULL==0.
mmap(0,..., MAP_FIXED...
char *p = NULL;
*p = 'a';

address 0 now contains the letter 'a' and the program does not fault.

The idea is that if you do the mmap in userspace, fill that area with specially crafted information, then cause the kernel to hit a bug in the software (in this case a pipe) which dereferences a NULL pointer, you can make it operate on data you control. This is due to the fact that userspace and kernel operate in the same virtual address space. Sometimes (often?) this means you can take over the box.

mmap_min_addr tries to address this problem by disallowing a program from mapping the low virtual address space. That means it ONLY affects the class of bugs that involve a NULL pointer. If it's a corrupt pointer instead a NULL pointer mmap_min_addr is totally useless. For either class of bugs hardware segmentation could help. It wouldn't be a security panacea, but it would eliminate the need for the specific hacks which both Linux and BSD use.

I thought we discussed this in July [slashdot.org]? Or is this a different exploit?

I think it's pretty clear that De Raadt and others have been discussing this vulnerability for quite sometime. On a list of affected systems [securityfocus.com], you can see it's been known on that site since August. Here's another fix discussed [kernel.org] that involves setting PER_CLEAR_ON_SETID mask to MMAP_PAGE_ZERO and that's from July (unfortunately, as the Register article said, that might cause problems with applications). In fact I think Spengler has bee

So it's really a bad design on Intel chips, that all operating system have to work around, except Windows which requires it to work like this......and on most Linux and BSD systems it will not work even if unpatched... and it has already been patched... and you need to be able to get a user on the local machine to run an app to exploit it (most users install from the Package Manager and so will never run a random downloaded app), and this does not appear to be a remote exploit?

What do you mean Windows requires it to work like this? On Windows accessing a NULL pointer is always an exception, no process is ever allowed to map the bottom page of memory. This has been true since Windows has existed. So in fact it is only Windows systems that are immune to this class of exploit because writing programs and kernel code vulnerable to it leads to an immediate crash.

If you wanted to specify this invariant on Linux you could, you'd just break some existing apps that depend on it. Ironically, it seems that Wine depends on this behavior.

Well, there's always MITRE Common Vulnerabilities and Exposures [mitre.org], which is a good pretty much dupe-free index of reported vulns. Most professional discussions of vulnerabilities tend to use CVE references.

For instance, this particular vuln looks like CVE 2009-2695 [mitre.org]. The one discussed in the July/. article appears to be CVE 2009-1897 [mitre.org].

The CVE pages are pretty good, complete with cross references to discussions and some pretty detailed analysis of the vulnerability.

I believe the vuln being talked about here is actually CVE 2009-3547 [mitre.org] (though full details aren't yet available on the MITRE site). If you google that number, you'll find the Red Hat patches.

OTOH, CVE 2009-2695 also googles to a RH bugzilla page, Bug 517830 [redhat.com], " kernel: SELinux and mmap_min_addr". The description on that page sounds suspiciously like the subject of current discussion. This page also has RH patches, in this case in two different RH Security Advisory links.

The problem we are discussing today is CVE-2009-3547. The root cause of the vulnerability is a null pointer dereference in pipe.c. The proper fix is to patch the kernel. It just so happens that if vm.mmap_min_addr != 0 then the bug cannot be exploited to gain root access. Therefore, mmap_min_addr can be used as a stopgap measure until you are able to patch the kernel. It also happens to be a good idea to leave mmap_min_addr set to a non-zero value to guard against other null-pointer dereference bugs th

No, it's not obvious at all. Which is why a bunch of people are posting instructions on how to do it. Actually, did either of the posts you were replying to use the term "obvious" or "user error"? I sure didn't see it.

I saw people posting relatively clear instructions to protect yourself until a proper fix is released.

I see the same thing on a lot of bugs, be they Windows or Linux or Mac or iOS or whatever. Someone comes up with a way to mitigate the risk and publishes that method to reduce the number o

If the result is non-zero the vulnerability doesn't exist.'Most deployed versions of linux'?.
So far only some unpatched RHEL versions allow this local exploit, even the Centos rip-off doesn't have it.

I'm not a real security guy, but my experiences with security bug reporting shows that nearly all such subtle bugs are pooh-poohed by the original authors till the exploit writer resorts to petulant scaremongering. I'm not sure which one is to blame for either one's behaviour.

All of these attacks IIRC require you to be able to mmap() page zero. Which is why mmap_min_addr is almost never set low enough in a decently protected OS. But the fact is that the exploit is a valid bug for a system which hasn't got that set to 4k. And there is a valid root exploit using pulseaudio (*ouch*) as a vector.

Linus might have been right in saying setuid is a 'vulnerability', but to call it a design flaw is wrong. Setuid is not a design flaw,
it is a trade-off - needed for something as simple as 'ping' to function (yeah, ping's got setuid, check it).

Being able to exploit a setuid binary after mmap'ing page zero with executable shell code, via a phpbb vulnerability which is exposed because of lack of php filtering is like saying... "look, having arranged these six dominoes, I only need to push *one* over".

I'm not denying either of them aren't right in their own way - but invariably original author vs security researcher sets up a very immature exchange of insults (and the ego of both types don't help either).

Setuid is certainly a trade-off, but it seems a little absurd that you need full root permissions to access just the special resources "ping" needs to function. If anything, vulnerabilities like these are calls for a more fine-grained capability-based security system, that only grants the expected privileges needed for a given process to function.

While I'm dreaming I'll also take sandboxing for user-executed processes. And a pony.

Setuid is certainly a trade-off, but it seems a little absurd that you need full root permissions to access just the special resources "ping" needs to function. If anything, vulnerabilities like these are calls for a more fine-grained capability-based security system, that only grants the expected privileges needed for a given process to function.

You are aware that in order for ping to work at all, it needs raw sockets so that it can write ICMP packets? Those are restricted because they allow you to spoof all sorts of network traffic (e.g., the ethernet address to IP address mapping) Which Would Be Bad.

The only way to remove the setuid requirement from ping (apart from making your system thoroughly insecure) is to allow messages to be sent and received on raw sockets opened by non-root only if they're ICMP ECHO messages (I'm not aware of any other I

You are aware that in order for ping to work at all, it needs raw sockets so that it can write ICMP packets? Those are restricted because they allow you to spoof all sorts of network traffic (e.g., the ethernet address to IP address mapping) Which Would Be Bad.

This seems less bad than kludgy workarounds.

Network services should never trust that the packets sent to it are not forged. Ever. Session-based authentication If the network services were written with this caveat in mind (which can never really be eliminated anyways, since there's no way of knowing whether the client app is mangling packets) then there would be no problem letting userland programs have access to raw sockets.

The only way to remove the setuid requirement from ping (apart from making your system thoroughly insecure) is to allow messages to be sent and received on raw sockets opened by non-root only if they're ICMP ECHO messages (I'm not aware of any other ICMP messages that it's useful for user code to send).

That's absolutely not the only way. You can make raw sockets accessible via a node in/dev, which you can assign to a group, control membership in, and setuid/setgid a NON-root user to "ping".

A *lot* of system resources are controlled in this manner (dri, sound, disks). I still don't think it's a sufficiently versatile security model (cf my comment on sandboxing), but it's a good place to start.

You are aware that in order for ping to work at all, it needs raw sockets so that it can write ICMP packets? Those are restricted because they allow you to spoof all sorts of network traffic (e.g., the ethernet address to IP address mapping) Which Would Be Bad.

It could be threatening to the network (which should not trust individual nodes much anyway, but many do), but not to the system itself... At least, I don't see how...

And yes, it's dueling egos. Theo is a very good coder, and OpenBSD is an amazing system, but Theo should stop talking to the public. It never helps. (Even when he's right, which he usually is when the discussion involves something technical.)

Theo is in charge of a BSD-based kernel that is only concerned with security, while Linus is in charge of a kernel that has to accommodate a much wider audience (like people who want to run Wine), and, of course, since both of them also have largish egos, they've both managed to say some silly things about each other's kernel...

Basically, unless you're already a Linus or Theo fanboy, their 'bickering' is not that important.:)

That does not look like a kernel problem to me at all. He's running a setuid program that allows the user to specify its own modules. And then you people are surprised he gets local root?

Am I missing something? Torvald's reply actually sounds pretty reasonable to me here. It might be nice if this exploit could be patched, but it seems a little preposterous to me that you could make that work in a way that doesn't leave an exploit. I'd say you need to be locking down your suid binaries more, not blaming kernel management.

Torvalds is absolutely correct, the whole issue is idiotic and isn't really an "exploit" because it doesn't result in ANY privilege escalation. (You must be root to run an exploit that gives you, um, root!) However, ego-starved security people get to jump up and down thumping their chests about how they h@x0r3d an extremely common Linux distribution and get their names splashed all over the press. Yeah, and instead of mmapping NULL, you can just 'insmod/my/evil.ko' and get the same effect. Sigh.

Those of us who know anything about security just ignore these "researchers". There are far more important bugs - even far more important security bugs - than a root-to-root non-issue. I'll happily patch my system with the next set of kernel updates, but I don't intend to grab it any sooner. It's more important to me that my system stay up and avoid regressions than fix a root-can-get-root non-exploit.

Before people jump on Theo's comment, it's worth pointing out that it was Linus who first described the OpenBSD developers as "masturbating monkeys". That said, it's still bloody childish irrespective of who it's coming from.

Is there some sort of exploit code I can run to check if my system is vulnerable? I tried to find some online, but I only came up with some code for SCO Unix [packetstormsecurity.org] and some code [grsecurity.net] that is so horrendously long that I don't dare running it for fear it might do something I don't want to happen on my system.

Then I read Torvald's comment about the Linux exploit, with Torvald referring to the openBSD developers as being __like__ a "bunch of masturbating monkeys".

Ok, so is this like some kind of secret code used among OS kernel developers? Like saying "my shoe is blue but the cow is hungry" really means "Oh man, this code is leaking memory and crashing my system"? Or is this some kind of secret initiation thing, where in order to truly become a member of the OS development club, you have to first... masturbate a monkey??!! Can somebody explain it, or maybe do some investigative reporting on this?

Linus's comment:
"That does not look like a kernel problem to me at all. He's running a setuid program that allows the user to specify its own modules. And then you people are surprised he gets local root?"
Sounds reasonable to me.

Linus's comment:"That does not look like a kernel problem to me at all. He's running a setuid program that allows the user to specify its own modules. And then you people are surprised he gets local root?"

Sounds reasonable to me.

Well, here's the thing... For the exploit to work you need either mmap_min_addr to be 0, or you need your process to have CAP_SYS_RAWIO. In other words, if you were running on a system that had mmap_min_addr set to 0, you could run this exploit without already having root authority. Wine needs this, apparently...

The workaround for mmap_min_addr (by exploiting dangerous SUID code in Pulse) was just icing on the cake.

Just checked my installations (Debian with custom kernel from kernel.org). They are all at 4096 for mmap_min_addr (and hence not vulnerable), but I seem to be unable to find a place where this is set. Does anybody know whether this is the kernel default?

I've felt for a long time that giving someone local shell access to a machine is never going to be completely secure. There's just too many degrees of freedom available, and too many different things that need to be secured. This is just another proof of concept of that principle.

With all the various different interfaces we have today, shell access is something only a small percentage of people need. Even those you could likely limit down to a few administrators, some programmers, and possibly a few spec

``I've felt for a long time that giving someone local shell access to a machine is never going to be completely secure.''

Indeed, it is not. There are various nasty things a regular user with shell access can do.

``This is just another proof of concept of that principle.''

No. This is a vulnerability that makes things worse than they should be. There are various ways to limit what regular users can do. A vulnerability that allows these users to become root means the system is broken, so that users actually get

You are trolling since you're trying to imply that this is the first time a flaw was found in the Linux kernel. This has happened numerous times, and it has been posted on/. numerous times, and there's always some fucking troll saying "If this was Windows...". Who the fuck cares!? Why make everything into some political bullshit with extremists throwing rocks at eachother?I'm going to drop a bomb here and it might shock you all, but it's the truth, here it goes: Linux is not perfect nor did anybody

Bit of a sweeping statement for a single case. So open source software isn't perfect either, course not, it's programmed by monkeys in shoes like all other software. But if you look at the statistics, even weighting for market scale, open comes out on top. This is actually a example of why, exploits are easier to find as researchers can look at, debug, pull apart, the source, unlike when it's a closed box. The fix will be done quickly (maybe in multiple ways by multiple people, until one stands out as best)

Actually, I was not trolling. Put simply, if this EXACT BUG was discovered in Windows, OS persons would be jumping about like grass-hoppers that THIS could never happen in OS software.

As pointed out elsewhere on these threads, most Linux versions already prevent this bug through a kernel memory map setting. However, WINE, which needs to run things in a Windows-compatible way, doesn't work with that setting, so it's turned off, and people with WINE installed remain vulnerable.

Hah, this just shows how EFFICIENT Linux is. Until recently, Windows achieved their local privilege escalation vulnerability rollout by having almost every home user running as fully privileged administrator accounts all the time. Linux achieves all this through a small tweak to the kernel build system, thus getting this feature to 100% of Linux users without any manual intervention at all.

Lets compare apples to apples. Lets say you have an out-of-maintenance version of Linux (which would be some of the intermediate releases not in use by major distros) and your distro won't be patching you. You have the option of paying someone a lot less than $400 to back-port this patch to your kernel version or doing it yourself.

Now lets say you have an unsupported version of Windows (like NT 3.51) and you find out there's a major security hole in Vista dating right back to 3.51. How are you going to get that fixed? If you answer "by upgrading" then use the same answer with Linux.

The whole point of FOSS is that you're never stuck, you can always just do it yourself or pay someone to do it for you, the vendor can't lock you out of the code running on your systems.

Of course, if you run a maintained version of any Enterprise Linux I'd put good bets down that they'll be patched shortly. If you spun your own distro, then you made the choice to maintain it yourself anyway.

Might be feeding the toll but,
Yup, randomly, anonymously taking your anger out on uninvolved bystanders is definitely the way to correct the system. I guess it never occurred to you that you are doing the same thing that put you in your little temper tantrum to begin with. Let's hope the people you target are more mature than you.

A patched kernel will fix this particular bug, but as long as wine is installed and mmap_min_addr is being set to 0 you have no protections for the next bug. Unless wine can be changed (which some smart people have told me it can't), there's nothing you can do except accept a higher security risk on your system.

Here's a fun tag line, "Windows applications can even make Linux less secure!"

The bug is fixed in the RC, but it's not exploitable in older kernels except when vm.mmap_min_addr = 0. I'm running the vanilla 2.6.30.3 kernel on Gentoo and it's not vulnerable, so it seems likely that you're only vulnerable if your distro (or you, if building a custom kernel) decided to change the default value.